This invention pertains to apparatus and methods for generating reference voltages in electronic circuits. In particular, the invention relates to integrated circuit implementations of voltage reference circuits that are applicable to implantable medical devices such as pacemakers and cardioverter/defibrillators.
Implantable medical devices, such as cardiac pacemakers and cardioverter/defibrillators, employ electronic control circuitry to enable the device to appropriately respond to sensed cardiac events and lapsed time intervals. A necessary component of such circuitry is a circuit for producing a fixed reference voltage. One type of circuit that is commonly used for providing a fixed reference voltage is the so-called bandgap voltage reference. Unlike other voltage references, such as Zener diodes, this circuit generates a reference voltage that is independent of temperature. Basically, as will be described in more detail below, a bandgap voltage reference generates a temperature independent voltage by adding together two voltages with opposite temperature coefficients. The circuit utilizes a current source with a positive temperature coefficient, referred to herein as a proportional-to-absolute temperature (PTAT) current source, to generate one of the voltages by flowing the current through a voltage converting resistor. In order for the circuit to perform properly and generate a temperature independent reference voltage, the voltage reference circuit must be tuned so that the PTAT current source delivers a precise amount of current across a precise resistance. The current magnitude that the source must deliver and the voltage converting resistance value may vary, however, due to amplifier offset, current source mismatch, and variation in junction characteristics. Since these parameters cannot be determined until the rest of the circuit is manufactured, the usual method of tuning the reference circuit is to trim (i.e., adjust) a current controlling resistor and/or the voltage converting resistor after the circuit is manufactured until the desired current level and reference voltage is obtained.
The method described above for tuning the reference circuit presents a particular problem, however, with respect to implantable medical devices. In order to reduce both space and power requirements, the circuitry of such devices is fabricated on integrated circuit (IC) chips whenever possible. The components of the bandgap voltage reference circuit can similarly be placed on an IC chip with the exception of the trimmable current controlling resistor of the PTAT current source and the voltage converting resistor. This is because most resistive elements on an IC chip are diffused resistors that cannot be adjusted after they are fabricated. Prior manufacturing methods have therefore located the current controlling resistor of the PTAT current source and the voltage converting resistor off-chip where they can be laser trimmed after the circuit is manufactured. Locating these resistors off-chip, however, provide noise channels that can adversely affect the operation of the bandgap voltage reference circuit. Due to the very low output current and non-linear nature of the PTAT circuit, it is extremely sensitive to noise from various sources (e.g., external electromagnetic interference, capacitively coupled current) that enter the circuit through the connections of the external trimmable resistors. This has necessitated the use of expensive and cumbersome EMI shielding in implantable medical devices to overcome the problem. It is a primary objective of the present invention to provide a trimmable resistor for a reference circuit that does not increase its noise susceptibility.
In accordance with the invention, a digitally trimmable resistor is provided as a current controlling resistor and/or a voltage converting resistor as part of a bandgap voltage reference circuit. The trimmable resistors, along with other components of the bandgap voltage reference circuit, may then be fabricated on an integrated circuit chip. Fabricating a resistor as an on-chip digitally trimmable resistor allows it to be trimmed after manufacture and greatly improves the noise immunity of the reference circuit as compared with an off-chip resistor. Such a bandgap voltage reference circuit fabricated on an integrated circuit chip includes a current source for generating an output current with a positive temperature coefficient and an on-chip digitally trimmable current controlling resistor for determining the magnitude of the output current and/or an on-chip digitally trimmable voltage converting resistor. The circuit further includes a p-n junction voltage source which has a negative temperature coefficient, where the negative coefficient voltage is added to a positive coefficient voltage derived from the output current of the current source to produce a reference voltage.
In one embodiment, an on-chip digitally trimmable resistor is implemented as a resistor network connected to a switch array. The resistor network comprises a plurality of individual series connected resistors, with each resistor connected in parallel with a switch of the switch array. A switch closure therefore shorts the current path around the resistor to which the switch is connected and electrically removes the resistor from the network. The state of the individual switches of the array thus determines the total ohmic resistance of the resistor network. By implementing the switch array as, for example, an array of MOSFET transistors, the switch states can be determined by the gate voltages applied to each individual resistor. The digitally trimmable resistor may then be trimmed so as to result in the desired output current from the current source and/or desired reference voltage after manufacture of the integrated circuit chip. In a presently preferred embodiment, the gate voltages of the MOSFETs are controlled by the bit lines of an electrically erasable programmable read-only memory (EEPROM). The ohmic value of the digitally trimmable resistor is then determined by the values programmed into the memory locations of the EEPROM.